Automotive body panels can be made by sheet metal stretch forming processes that use complementary, double action forming tools in a press and the pressure of a working gas to stretch form a preheated blank against the forming surfaces. In one embodiment, the process is applicable to stretch forming of a superplastically formable or quick plastically formable metal alloy blank into a sheet metal product of complex shape. The metal alloy may, for example, be a magnesium-containing, aluminum alloy having a fine-grained microstructure (grain size suitably less than ten micrometers) for high elongation plastic forming. Typically the aluminum alloy sheet has a thickness in the range of about 0.7 to 4 mm. The sheet metal blank is given a preform shape involving substantial elongation of the sheet. In a second action of the tools the preform is then shaped into the final product. Such a process is described in U.S. patent application Ser. No. 10/274,493, filed Oct. 17, 2002, entitled “Gas Pressure Preforming Double Action Superplastic or Quick Plastic Forming Tool and Method”, and assigned to the assignee of this invention. That specification, including the drawing figures, is incorporated by reference into this application for its description of the two-stage forming process.
The method is particularly applicable to forming the sheet metal into a stretch formed product of complex three-dimensional curvature and regions of sharp corners and high elongation. For example, the invention is applicable to the forming of automotive vehicle body panels.
In accordance with two stage forming using gas pressure, the sheet metal is usually formed in a single press using complementary, but not mating, heated forming tools. The tools are in opposing (facing) relationship and movable from an open position, for insertion of a sheet metal blank, to their forming positions. Preferably, the blank is externally preheated to a desired forming temperature. After insertion of the preheated blank, the tools are moved to a first stage preforming position. The edges of the blank are gripped by a binder mechanism and gas pressure is applied to one side of the heated sheet to stretch it against a preform tool surface. The opposing, finish-shape tool is then moved closer to the preformed sheet in a second stage forming position. Gas pressure is applied to the opposite side of the sheet to force it back against the finish-form tool to complete the shaping of the sheet metal part. The press is then opened for removal of the formed part and insertion of a new blank.
The preform tool is shaped to accomplish a major portion of the stretching and elongation of the sheet in forming it toward the final part shape. The finish tool completes bends and recessed corners and defines the final detailed shape of the sheet metal produced in this press operation. In each forming stage, the pressure of a suitable working gas, such as air or nitrogen, is used to push and stretch the sheet against the respective tool surfaces. The pressure is applied to opposite sides of the sheet in the successive preform and finish-form steps. Thus, the necessary elongation lines or stretch directions in the sheet to form the part are predetermined. A substantial part of the elongation is accomplished in the preform step and is introduced nearly evenly over the preform shape. The final elongation is accomplished by forcing the preformed sheet away from the preform tool against the shaping surfaces of the finish shape tool.
This stretch forming process is efficient in its utilization of a single press with two forming tools. However, the working gas must be applied and vented from each side of the metal workpiece and the forming must be done at a strain rate that does not introduce defects in the visible surface of the formed part. The overall process has remained slow for high volume production operations. Accordingly, it is an object of this invention to increase the forming speed of the two stage stretch forming process and minimize localization of the strain in the formed part.